Soil contamination by deposits from the atmosphere and especially acidic components which deposit directly, indirectly or in a form in which the acid component is generated upon the ground, has long been recognized as a serious problem. These acidic components arise in the atmosphere in various forms, primarily as acid-forming gases such as sulfur oxides and nitrogen oxides. However, they also may be emitted into the atmosphere and deposited from the atmosphere or may be present in the atmosphere as free acids, such as hydrochloric acid, sulfuric acid or nitric acid and hydrogen fluoride.
The deposits collect upon the ground in gas, liquid or particle form and may be carried onto the ground with rain, snow or with mixed wet precipitation. As early as 1872, Smith was able to describe the phenomenon of and coin the term "acid rain".
Measurements taken of the soil at various locations has shown an increasing acidity which has been traced to increasing acidity of the atmosphere and an increase in acid emissions into the atmosphere.
The primary sources of such acidic deposits are sulfur-dioxide (SO.sub.2) and the nitrogen oxide (NO.sub.X) compounds which are generated in the combustion of gaseous, liquid and fossil fuels and organic fuels generally, utilizing air. The acid compounds are also released in less quantities from metallurgical plants during the roasting of ores and from various chemical process or other emissions.
In the northeastern United States, acid formers are constituted of 65% by volume of sulfur-containing acid compounds and 30% nitrogen-containing acid compounds, the balance of 5% being hydrogen chloride or hydrochloride acid. This ratio has tended to shift in favor of increasing proportions of nitrogen-containing acid as emission control standards are increasingly applied more successfully to the combustion of sulfur-containing fuels, thereby reducing the sulfur dioxide emissions or at the very least, stabilizing them.
Typical measurements in Europe for the pH value of rain range from 3.8 to 4.8. Hoarfrost in municipal areas has been shown to have a pH value of 3.2 and fog in industrial centers a pH value of 3.0. Pure water in equilibrium with the average of carbon dioxide content of air (0.03% by volume) has a pH value of 5.6. From these values it will be apparent that emissions have given rise to a sharp increase in the acidity of deposits from the atmosphere upon the ground. These acidification deposits have been found having a direct and indirect effect on vegetation. In various areas of Central Europe for example, the pH value of the ground has diminished to 4.3 whereas in northeastern North America a pH value as low as 4.1 has been measured.
In practice, pH values of 5.6 to substantially higher and even neutral values are preferred for most plant growth.
A direct result of the acidification of the soil and the ground environment is the leaching out of valuable nutrients. There is also at least a possible direct effect on the exposed surfaces of plant tissue. This has been attributed primarily to the effect of sulfur dioxide, nitrogen oxide and gaseous fluoride compounds.
For example, in pine and evergreen forests the life of needles has been sharply reduced, regeneration of replenishment of needles has been minimized and a filter effect in the crown region of the plant has shown that acid rain is able to reduce the pH of the soil in such stands of evergreen well below the pH values in open fields. This has been attributed at least in part to the fact that in evergreen stands and the like, dry-deposited acid formers can be washed out additionally.
The high hydrogen ion concentrations, apart from causing tissue damage can also mobilize surface deposits of heavy metals which can themselves be detrimental to plants. In hilly terrain at levels of 600 to 800 meters above the base of a mountain or hill, because of the solar radiation effect and the photo-oxidation of NO.sub.X by ozone, peroxyacyl nitrate forms, where there is more than about 0.00005 mg SO.sub.2 /m.sup.3 of air, the plants are more strongly damaged than with the NO.sub.X and SO.sub.2 alone.
An indirect effect upon the ground, of the acidic deposit is the leaching of calcium, magnesium and potassium, all important plant nutrients, from the soil.
Simultaneously, toxic elements (heavy metals) and aluminum may be mobilized to cause substantial root damage. The reduction of the pH value of the ground and the increase in the concentration of available heavy metal ions, have a negative effect on the growth rate and on the activation of soil microorganisms which are essential for effective plant growth.
The detrimental effect of such acid deposits, in sum, can be manifested in massive forest death, which has already been observed throughout at least northeastern North America and in Europe. More recently, however, the effect has also been observed in the reduced growth rate of crop plants such as tomatoes and damage to other plantings such as barley.